This invention relates to a light source apparatus using a flash discharge tube, and more particularly to a light source apparatus in which the light emission of a flash discharge tube is controlled by a semiconductor switching element connected in series with the flash discharge tube.
Recently, a flash discharge tube light emission control circuit has been developed which uses a commonly termed serial control system including a semiconductor switching element connected in series with the flash discharge tube. This circuit also includes a thyristor connected in parallel with the flash discharge tube and an inverting capacitor provided between the thyristor and switching element. The inverting capacitor is charged when the thyristor is triggered by a light emission interruption signal. With the charging of the capacitor the switching element is rendered non-conductive to interrupt the light emission of the flash discharge tube. However, even with this construction, current for charging the inverting capacitor flows through a loop formed by a main capacitor, the flash discharge tube, the inverting capacitor and the thyristor after the switching element is rendered non-conductive, thus charging the inverting capacitor and causing residual light. Also, in a light receiving element for receiving light reflected by a foreground subject, a delay time is involved in the operation of the photoelectric conversion due to the presence of a coupling capacitor for the light receiving element. This leads to a drawback that by the time when the light emission interruption signal is produced the film has already been exposed beyond the proper exposure level. This effect of the time delay is particularly pronounced in case when the light emission period is short, i.e., at the time of the short distance photographing. Also, the extent to which the excessive exposure results varies with the sensitivity of the film.
An object of the invention is to provide a light source apparatus using a flash discharge tube based upon the serial control system, in which the excess of exposure due to the residual light from the flash discharge tube or delay time involved in the photoelectric conversion of the light receiving element is compensated for according to the film sensitivity.